1. Field of the Invention
The present invention relates to the field of metal matrix composite materials and relates more particularly to an insert made from ceramic fibers in a metal matrix to reinforce a metal part.
2. Description of the Related Art
In the field of aeronautics, in particular, it is a constant goal to optimize the strength of the parts for a minimum weight and size. Some parts may now comprise an insert made from a metal matrix composite material, hereinafter referred to as MMC, and the part may also be monolithic. Such a composite material comprises a metal alloy matrix, for example of titanium Ti alloy, within which fibers extend, for example ceramic fibers of silicon carbide SiC. Such fibers have a much higher tensile strength than that of titanium (typically 4000 MPa compared to 1000 MPa). Hence it is the fibers which absorb the stresses, while the metal alloy matrix acts as a binder with the rest of the part, and also protects and isolates the fibers, which must not be in contact with one another. Furthermore, ceramic fibers are erosion resistant, but must necessarily be reinforced with metal.
These composite materials can be used in the manufacture of disks, shafts, cylinder bodies, casings, spacers, as reinforcements of monolithic parts such as blades, etc.
For compressor disks in a turbine engine, for example, a known reinforcement technique consists in inserting a circular winding of coated fibers in the part. A technique for manufacturing an MMC insert is based on the principle of winding coated wires described in patent EP 1.726.677 filed in the name of Snecma. The insert is obtained from a plurality of coated wires each comprising a ceramic fiber embedded in a metal sheath. This type of wire is henceforth designated as coated wire. The manufacture comprises a step of winding a bundle or a joined layer of coated wires around a solid of revolution, perpendicular to the axis of the part. The insert is then subjected to a hot isostatic compression step in a container. The term container means a blank of a metal part in which a cavity is machined for accommodating the MMC insert, and which is then subjected to a hot isostatic compaction treatment. Said treatment is hereinafter referred to as HIC.
The parts described thus obtained are of the circular type and, in addition to compressor disks, are particularly suitable for the production of circular parts such as shafts, cylinder bodies or casings.
Other mechanical parts require different properties from those presented by the circular parts. This is the case in particular of the rods used, for example, in landing systems or structural parts such as engine suspensions, which are essentially oblong in shape. The function of these parts is to transmit a unidirectional tensile and/or compressive force. The reinforcement of these parts accordingly requires MMC inserts that are at least partly rectilinear. This is because the fibers must be oriented in the stress direction.
The industrial manufacture of these inserts at minimum cost is a delicate matter.
A method is known for fabricating a mechanical part comprising at least one insert of MMC material. The method comprises the fabrication of a draft insert by winding a bundle or a joined layer of coated wires around an annular support, part of which comprises a rectilinear portion.
The method described in patent FR 2.919.284 in the name of SNECMA and Messier-Dowty develops this principle and then comprises the insertion of the above-mentioned draft insert in a first metal container, the hot isostatic compaction of the first container, followed by the machining thereof to form a rectilinear insert element. After the manufacture of this insert element, the method for manufacturing a mechanical part comprises the following steps: insertion of the insert element into a second container, hot isostatic compaction of the second container and machining of the second container to form the desired mechanical part. The mechanical part thus obtained, for example a rod, can be used advantageously to transmit the unidirectional tensile and/or compressive forces in the direction of the ceramic fibers which have been incorporated therein.
Instead of passing through the intermediate step of compaction of the draft insert, followed by its cutting into rectilinear insert elements, it is feasible to cut the annular reel forming the draft by ensuring the holding of the coated wires in a bundle.
Patent application FR 2.925.896 teaches the incorporation of this type of bundle in a rectilinear groove terminating at its ends.
This solution has many drawbacks which impact the industrialization of these operations:                The coated wires are lost in the non-straight portions. This loss is not negligible because the coated semi-finished wire product represents a substantial share of the total cost of the part.        The winding, particularly on oval shapes, generates stresses in the wound insert that are liable to be released while causing a deformation of the insert during the cutting operation.        
These techniques require a large number of systems for holding the coated wires at the cutting zones.